228 PROCEEDINGS OF THE ACADEMY OF [1881. 



lacral vessels entering the body through openings in the test, whence 

 they are continued beneath the vault by means of tubes. 



Digestive apparatus composed of a convoluted sac, surrounded 

 by a delicate calcareous network. 



Column long; its cross-section circular, elliptical, sometimes 

 ptentagonal or quadrangular; central perforation small to medium, 

 rarely large. 



A. Sub-family PLATYCRINIDJE Roemer. 

 (Amend. Wachs. & Spr.) 



The name Platycrinidiae has been used by most writers in a full 

 family sense, and in this they seemed to be justified, as most of 

 the genera are by their general aspect readily distinguished from 

 those of the Actinocrinidie and Rhodocrinidae. The differences, 

 however, which produce that particular habitus, are evidently not 

 tlie result of marked anatomical modifications. 



The body of the Platycrinidaj, according to the views of other 

 writers, is composed only of basals, primary radials, and vault 

 pieces, all succeeding plates in a radial direction are considered 

 by them to be arm plates. The rays in this group generally 

 become free from the first axillary, but the extended parts are 

 true extensions of the body, covered like this by regular vault 

 pieces, and these arranged in the same manner, they are not arms 

 in our sense, as they possessed no true articulation. If the 

 respective parts in the Actinocrinidse are to be regarded as 

 radials, then also are those of the Platycrinidse, they compose in 

 the former the sides of the body walls only in adult specimens, 

 in the younger state they form free appendages as in the mature 

 Platycrinidffi. The plates of the extended parts are joined by 

 suture, there is no hinge line, and the articulation was by ligament 

 only, probably similar to that of the anal tube, which certainly 

 was flexible to some extent. 



The distinctions between Platycrinidse and Actinocrinidffi are 

 more readily perceived than described, and seem to be fairly ex- 

 '^ressed by saying that the former represent a younger stage of 

 = the latter, and remained as a persistent type of that stage of 

 growth. The interradial regions are represented by a single 

 plate, leaving the upper radials unconnected laterally as in the 

 j'oung Actinocrinoid. 



